1. Field of the Invention
The present invention relates to a process for the separation of isobutene from normal butenes in a mixed C.sub.4 refinery stream. More particularly, the invention relates to a process where the butene-1 in the stream is isomerized to butene-2 to facilitate the separation. More particularly, the invention relates to a process wherein the isomerization of butene-1 to butene-2 and the separation of the butene-2 from the isobutene takes place in a distillation column reactor.
2. Related Information
Isobutene in some utilizations such as oligomerization or polymerization must have a high degree of purity, that is, be substantially free of other C.sub.4 isomers such as butene-1 or butene-2. The separation of commercial quantities of isobutene from butene-1 by fractional distillation is quite difficult because of their close boiling points (isobutene -6.9.degree. C.; butene-1 -6.3.degree. C.). On the other hand butene-2 which boils at 1.0.degree. C. (trans butene-2) and 3.7.degree. C. (cis butene-2) is much more easily separated by distillation.
In order to achieve the requisite purities, processes have been developed such as etherification which reacts isobutene then recovers the ethers, followed by disassociation of the ethers to produce isobutene and alcohol which are easily separated by distillation (see U.S. Pat. Nos. 4,447,668; 4,482,775; 4,551,567 and 4,691,073).
It has been known for some time that olefins can be isomerized under mild conditions using a catalyst of palladium oxide supported on alumina in the presence of hydrogen. The actual active catalyst is probably palladium hydride which is produced during operation.
As commercialized, hydroisomerization is a process used to upgrade C.sub.4 streams, usually from fluid catalytic cracking units. In the fixed bed process as practiced by some, butadiene contaminating the feed is hydrogenated to butenes, and the butenes are isomerized to the equilibrium mixture which is predominately butene-2. The advantage of that process is to remove butadiene which causes the loss of acid used in the alkylation process and improvement of the alkylate octane number in HF alkylation by using mostly butene-2 in the feed rather than butene-1.
Palladium catalysts are known and used for the butene-1 to butene-2 isomerization. As a matter of fact, one source, IFP, does not recommend palladium because of its activity for use in streams where butene-1 is to be recovered.
According to the literature, isomerization occurs only after hydrogenation of the butadiene. In the fixed bed processes a three to four percent relative loss of butene occurs due to hydrogenation as the isomerization is pushed toward equilibrium.
The use of catalytic distillation processes is known in the art. See for example the series of patents assigned to Catalytic Distillation Technologies including U.S. Pat. Nos. 4,215,011; 4,232,177; 4,242,530; 4,302,356; 4,307,254; 4,336,407; 4,439,350; 4,443,559; 4,482,775; 4,504,687; 4,510,336 and 4,536,373. Catalytic distillation has been used in the isomerization of C.sub.4 alkenes as noted in U.S. Pat. No. 4,482,775 listed above. However, such a process used an acidic cationic exchange resin catalyst to produce iso and normal butenes. Arganbright in U.S. Pat. No. 5,087,780 discloses a process for the isomerization of butene-2 to increase production of butene-1 in a distillation column reactor using a palladium catalyst supported on alumina.
An advantage of the present invention is the separation of isobutene from normal butene in a high degree of purity. Another advantage sought to be achieved is relatively higher production levels of butene-2 which can be more readily separated from isobutene. Another benefit would be to convert butadiene to butenes.